Put it this way - my Father has lived in the same house for 63 years. For all of this time his hot water has been supplied solely by immersion heater (no gas boiler or central heating).
I am now in my 50's and I lost count many years ago of the number of elements that I watched him change and then later did them for him. He used to buy them 2 at a time and there was always a spare element and a spanner kept in the airing cupboard ready for the next change. They would generally last 18 - 24 months before going bang and it would be necessary to get a long scraper and pull as much of the loose scale as possible through the hole before screwing in the replacement.
The scale reducer was fitted at the same time as the last element and that was at least 12 years ago. It would be a bit of a coincidence if the water composition changed radically at that exact moment in time.
Or am I deluding myself?